If you're filing based on your having been a green card holder for more than five years, it's okay if you put "unknown "for your husband's date of naturalization. It would also be okay to put "cannot be determined at this time" . You can either attach an addendum to form N-400 explaining, or explain in person during the interview That your husband is withholding that information from you, because you suspect he does not want you to become a citizen.
Behar Intl. Counsel 619.234.5962 Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.